I'm fairly sure I know what's going on here. I'm assuming the injector pictured is the one nearest the intercooler pipe? On earlier HDi 110's, the intercooler pipe incorporated an air doser (butterfly valve essentially) which opens/closes depending on how my EGR is required. This will create a small vacuum in the manifold and forces exhaust gasses to be recirculated, preventing clean air entering the intake. This air doser commonly leaks, meaning any oil residue in the intercooler pipework will drip out and settle in that little indentation near injector 4 (or 1?). My car had this issue, and I simply replaced the metal 90deg bend with one from later cars. See below:

Disabling the EGR with a blanking plate and removing the air doser unit altogether and replacing with the pipework from a later car (easy job) is the best solution. You'll also see more power and better mpg, as well as knowing your engine is no longer sucking in dirty exhaust gases.

To remove the EGR and air doser, you need to both install a blanking plate and plug the vacuum supply to the EGR electrovalve. These EV's are mounted on the bulkhead, on the 110 there are 3 - the one you're after is the one with the blue plug. Locate the vacuum line going into it from the vacuum pump, and plug it with a suitable screw/bolt ensuring it's a tight seal to prevent any air leaks.
Next, remove the 90deg elbow and replace it with one from a later car (pictured in my last post), there are two bolts which hold it onto the intake manifold and it's at that join where the EGR pipe enters the intake manifold, so while replacing it you may as well install the blanking plate (use some high temp liquid gasket to prevent boost leaks). Plug the vacuum line that used to go to the air doser and that finishes the install!

If you decide not to remap the car with a no-EGR map, you'll find a dormant code stored on the ECU (which wont trigger an EML) saying "air flow higher than expected". Just ignore it

Yup and very easily! It sounds a bit complicated but it really isn't, if I can do it I'm sure you can too! The only tricky but is the bolts (well studs, technically) that hold the elbow onto the intake manifold. Access is a little tight so be careful not to drop anything (like I did ) as you'll have to take a trip to the dealer for new ones!